The present invention relates to pattern defect inspection methods and apparatus using a laser beam as illumination light, mainly for inspecting and observing micro pattern defects or foreign matter contamination occurring in manufacturing processes for semiconductor devices and flat panel displays.
Circuit patterns tend to continually become finer and smaller as semiconductor devices become more highly integrated. Smaller and finer circuit patterns have spurred a demand for higher resolution when inspecting for defects of circuit patterns that have been formed on semiconductor wafers by photolithographic processes using photomasks or reticles. One technique for enhancing resolution when detecting pattern defects involves the use of illumination light on shorter wavelengths from visible light to ultraviolet light. Mercury lamps and xenon lamps, for example, have been conventionally used as illumination light sources, while only the required wavelengths are optically selected and utilized from among the various line spectra emitted from these lamps.
Illumination from a typical light source lamp, however, contains only a few line spectra in the ultraviolet region. A larger size lamp with higher power must therefore be used to obtain a sufficient light intensity, but this results in the problem of lower lighting efficiency. Yet another problem is that correcting the chromatic aberration of optical systems used for pattern inspection is difficult due to the wide spectral bandwidth.
Optical aligners of this type used in semiconductor device manufacturing also require high resolution. For this reason, optical aligners equipped with a krypton fluoride (KrF) excimer laser that emits light at a 248 nm wavelength are mainly used. Optical aligners using an argon fluoride (ArF) excimer laser that emits an even shorter 193 nm wavelength have also been developed. However, these excimer lasers are large in size and use fluorine gases that are armful to the human body, so specified safety measures must be implemented.
Recently, a great deal of attention is being focused on solid-state YAG lasers as another type of ultraviolet laser. YAG lasers are capable of generating a third harmonic (355 nm wavelength) or fourth harmonic (266 nm wavelength) by wavelength conversion when the laser beam is passed through a nonlinear optical crystal. This has led to the development of compact, easy to handle ultraviolet lasers. These compact and easy to use ultraviolet lasers are highly effective for use in a pattern inspection apparatus.
Laser beams have superior coherence, but this causes enhancement and attenuation in the light flux when they are used to illuminate a sample, and such illumination produces an interference fringe on the sample. In a pattern inspection apparatus using a laser, as disclosed in Japanese Patent JP-A No. 271213/1999, a light beam emitted from a laser light source is guided into a fly-eye lens (micro-lens array) to form a multi-spot light source. This multi-spot light source is focused so as to strike a sample under test so that the sample is uniformly illuminated with light. The intensity of the light reflecting from the sample is then detected with a charge integration type of CCD line sensor.
The aforesaid pattern defect inspection apparatus using a laser has the following problems.
The light beam emitted from the laser is transformed into a multi-spot light source by a fly-eye lens and is focused by a condenser lens so as to illuminate the entire area of the sample under test. The incident angle of the illumination light on the surface of the sample under test is determined by the focal positions of the fly-eye lens and the condenser lens. When a thin film is formed on the surface of the sample, the light reflected from the sample contains light components reflecting from the surface of the thin film and also light reflecting from the lower layer surface of the thin film after penetrating into the thin film. Thus, the phase of the light reflecting from the lower layer surface of the thin film changes on the surface of the thin film according to the thickness of the thin film, so that the reflected light intensity to be detected on the surface of the sensor will vary.
Now we will discuss how the intensity of reflected light changes in cases where a thin film, such as an insulating film, is formed on the surface of a sample. A typical interference model is shown in FIG. 6. Here, the wavelength of illumination light 37 is set as λ, the incident angle of the illumination light 37 relative to the normal line direction on the surface of the sample is θ, the refractive index of the air layer 34 is n0, the thickness and refractive index of the thin film 35 are t1 and n1, respectively, and the refractive index of the semiconductor substrate 36 is n2. If the intensity of light reflected 38 reflected from the surface of the thin film 35 is set as r01, and the intensity of light 39 reflected from the substrate 36 after passing through the thin film 35 is r12, then the composite reflected light can be defined as R. These factors can be theoretically modeled as Fresnel equations and expressed by the following equations 1 to 4. An example of the calculated results is shown in FIG. 7, wherein the horizontal axis represents the thickness of the thin film 35 and the vertical axis represents the composite light intensity R. Changes in the composite light intensity versus the film thickness, when plotted, result in waveform 40.                     X        =                              4            ⁢            π            ⁢                                                   ⁢            n1t1                    λ                                    (Eq.  1)                                r01        =                              n1            -            n0                                n1            +            n0                                              (Eq.  2)                                r12        =                              n2            -            n1                                n2            +            n1                                              (Eq.  3)                                R        =                                            r02              2                        +                          r12              2                        +                          2              ⁢                              r01r12cos                ⁡                                  (                  X                  )                                                                          1            +                                          r01                2                            ⁢                              r12                2                                      +                          2              ⁢                              r01r12cos                ⁡                                  (                  X                  )                                                                                        (Eq.  4)            
FIG. 8 shows a cross section of a sample on which circuit patterns are formed. A circuit 41 and a circuit 42 are formed on a semiconductor substrate 36, and the entire surface of the sample is covered with an insulating film 35. Assuming, for example, that the circuit 41 has a low density pattern, while the circuit 42 has a high density pattern, and also that the thickness of the insulating film 35 is not uniform for some reason, the thickness of the insulating film 35 will be tb on the circuit 41 and t11 on the circuit 42. As mentioned above, if the thickness of the insulating film 35 varies, then the light reflected from the sample, which contains a light component reflecting from the thin film surface and a light component reflecting from the thin film lower layer surface (after penetrating into the thin film), changes accordingly. FIG. 9 shows this change in the reflected light intensity caused by the example of FIG. 8. The difference in reflected light intensity between the thickness t10 and thickness t11 of the insulating film 35 corresponds to a portion 44 on a waveform 43, that indicates the relation between the reflected light intensity and the film thickness. A change (Rt1) can be observed in the reflected light intensity. When the pattern of the sample is inspected under this condition, the change in the reflected light intensity caused by the difference in the transparent film thickness is detected as a change in brightness.
In inspection methods used heretofore, a sample is illuminated with light incident on the sample at a certain angle. So, when the thickness of a transparent film formed over the surface of the pattern varies at different position's, the reflected light intensity from the sample, which contains light components reflecting from the surface of the transparent film and also light components reflecting from the lower layer surface of the transparent film, changes according to the position on the film, and so an interference fringe pattern occurs. Due to this interference fringe pattern, the reflected light intensity to be detected on a CCD line sensor varies according to the position on the thin film. To reduce adverse effects from uneven brightness or shading caused by the interference fringe pattern, the CCD line sensor must be adjusted so as to detect dark areas, and, as a result, the detection sensitivity is reduced to a lower level.